Computer modelling of the polymerization of actin shows the size of the nucleus for polymerization to be 4 and that the rate of nucleation of G-actin containing bound Ca-++ is slower than for G-actin containing bound Mg-++. At steady state, complete exchange occurs between actin monomers and actin polymers in the presence of 0.5 mM Mg-++. ATP hydrolysis is faster than actin exchange. The actin exchange is inhibited by substoichiometric concentrations of cytochalasins and when ADP is substituted for ATP. These data, and other data, may not be simply interpretable by any one model (such as treadmilling or exchange-diffusion) for equilibration. Covalently crosslinked actin dimer binds cytochalasin B weakly (K-D = 50 muM) relative to polymer and does not interact with profilin, which binds to monomer. The dimerpolymerizes with no detectable lag implying that the dimer is the nucleus in this case. Hydrolysis of ATP is slower than polymerization of dimer. It is concluded that polymerization of dimer and hydrolysis of ATP are not coupled and that polymer-bound ATP is hydrolyzed subsequent to polymerization